(a) Technical Field
The present disclosure relates to an on/off control method for an air blower of a fuel cell vehicle. More particularly, it relates to an on/off control method for an air blower of a fuel cell/supercapacitor hybrid vehicle, which can prevent the voltage of a fuel cell stack from exceeding a predetermined maximum voltage and improve vehicle acceleration response during switching-off of the air blower.
(b) Background Art
A fuel cell system is an electricity generation system that does not convert chemical energy of fuel into heat by combustion, but electrochemically converts the chemical energy into electrical energy in a fuel cell stack. Such a fuel cell system can be applied to the supply of electric power for small-sized electrical/electronic devices such as portable devices, as well as to the supply of electric power for industrial and household appliances and for vehicles.
One of the most attractive fuel cells for a vehicle is a proton exchange membrane fuel cell or a polymer electrolyte membrane fuel cell (PEMFC), which has the highest power density among known fuel cells. The PEMFC is operated in a low temperature and is able to start up in a short time and has a fast reaction time for power conversion.
The fuel cell stack included in the PEMFC comprises a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member, and a bipolar plate (separator). The MEA includes a polymer electrolyte membrane through which hydrogen ions are transported. An electrode/catalyst layer, in which an electrochemical reaction takes place, is disposed on each of both sides of the polymer electrolyte membrane. The GDL functions to uniformly diffuse reactant gases and transmit generated electricity. The gasket functions to provide an appropriate airtightness to reactant gases and coolant. The sealing member functions to provide an appropriate bonding pressure. The bipolar plate functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
The fuel cell stack is consisted of a plurality of unit cells, each unit cell including an anode, a cathode, and an electrolyte (electrolyte membrane). Hydrogen as a fuel is supplied to the anode (“fuel electrode”, “hydrogen electrode”, or “oxidation electrode”) and oxygen containing air as an oxidant is supplied to the cathode (“air electrode”, “oxygen electrode”, or “reduction electrode”).
The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by a catalyst disposed in the electrode/catalyst layer. The hydrogen ions are transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane, and the electrons are transmitted to the cathode through the GDL and the bipolar plate.
At the cathode, the hydrogen ions supplied through the electrolyte membrane and the electrons transmitted through the bipolar plate react with the oxygen containing air supplied to the cathode to produce water. Migration of the hydrogen ions causes electrons to flow through an external conducting wire, which generates electricity and heat.
As a vehicle equipped with the above-described fuel cell, there has been developed a fuel cell/battery hybrid vehicle or a fuel cell/supercapacitor hybrid vehicle, in which a high voltage battery or a supercapacitor is used as a separate power source for providing the power required to drive a motor in addition to the fuel cell as a main power source in a large vehicle such as a bus as well as a small vehicle.
Especially, a fuel cell/supercapacitor hybrid vehicle which does not employ a power converter has been studied, and the fuel cell/supercapacitor hybrid vehicle has many advantages such as high fuel efficiency (high regenerative braking, high efficiency of supercapacitor, and without the use of the power converter), an increase in durability of the fuel cell, high reliability control (automatic power assist and automatic regenerative braking function), and the like.
In the hybrid vehicle in which the fuel cell and the supercapacitor are directly connected, the fuel cell continuously outputs power at a constant level during driving. If there is surplus power, the supercapacitor is charged with the surplus power, whereas, if there is insufficient power, the supercapacitor supplies the insufficient power to drive the vehicle.
FIG. 1 is a diagram showing a structure of a fuel cell/supercapacitor hybrid system. Reference numeral 10 denotes a fuel cell, reference numerals 20 and 21 denote a supercapacitor and a supercapacitor initial charging device, and reference numeral 30 denotes an air blower.
FIG. 2 is a graph showing performance deterioration rates obtained by reducing the maximum voltage of a fuel cell stack to OCV (about 1.0 V), 0.95 V, 0.9 V, and 0.85 V. As shown in FIG. 2, fuel cell performance is less reduced with the passage of time if the maximum voltage of the fuel cell stack is lower and the recovery rate of regenerative energy during the switching-off of the air blower is increased.
FIG. 3 is a diagram showing an on/off control map of a conventional air blower. When the supercapacitor voltage is above V1, the air blower of the fuel cell system is switched off, and the supercapacitor prevents the voltage of the fuel cell stack from rising. At this time, the fuel cell/supercapacitor hybrid vehicle is driven only by the energy of the supercapacitor, and the recovery rate of regenerative braking energy is increased.
On the other hand, when the supercapacitor voltage is below V2, the air blower is switched on and, at this time, the voltage of the fuel cell stack is increased to supply energy to the vehicle. In this case, when the vehicle is decelerated, the supercapacitor is charged, and thus the supercapacitor voltage is increased. Subsequently, when the supercapacitor voltage becomes above V1, the air blower of the fuel cell system is switched off again.
However, the conventional system has the following problems. The conventionally system much exceeds the maximum voltage of the fuel cell stack (above 20 to 30 V). More particularly, if excessive regenerative braking occurs when the supercapacitor voltage is above V1 and the air blower is switched on, it is difficult to prevent the voltage of the fuel cell stack from rising by the supercapacitor, even immediately after the air blower is switched off. In addition, the conventional system shows a low vehicle acceleration response during maximum acceleration when the air blower is switched off. More specifically, the vehicle acceleration response is low during maximum acceleration, if the supercapacitor voltage is slightly higher than V2 (0 to 20 V) during the switching-off of the air blower. Moreover, even when the air blower is switched on, it takes about 1 to 2 seconds for the fuel cell system to operate normally.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.